The present invention relates to an ink jet printing apparatus for printing an alignment layer on a substrate of a liquid crystal display (LCD) and a method for printing an alignment layer using the novel apparatus. More particularly, the present invention relates to an LCD alignment layer printing apparatus having an ink jet head incorporating a structure for ensuring that the printed alignment layer has a uniform thickness.
A liquid crystal display (LCD) forms desired images by adjusting the amount of light transmitted through the panel in accordance with image signals applied to a number of control switches arrayed in a matrix on a substrate of the panel. The LCD comprises an LCD panel on which images are displayed by disposing a liquid crystal material between a color filter substrate and a thin film transistor (TFT) substrate, an LCD drive IC for operating the LCD panel, a back-light unit used as a light source of the LCD, and a chassis used to fasten respective components of the LCD into a single display unit.
During the fabrication of an LCD panel, an alignment layer, which is used to align the molecules of the liquid crystal material in a predetermined direction, is applied on an upper substrate, viz., the color filter substrate, and a lower substrate, viz., the TFT substrate, of the display. FIG. 1A is a schematic perspective view of an exemplary embodiment of a prior art LCD alignment layer printing apparatus, and FIG. 1B is a schematic perspective view of another exemplary embodiment of a prior art alignment layer printing apparatus.
The prior art alignment layer printing apparatus shown in FIG. 1A comprises an ink dispenser 141, a printing roller 142, a resin plate 143, an anilox roller 144, a doctor roller 145 and a stage 146. In this embodiment, an LCD substrate 147 is seated securely on the stage 146, and an alignment layer is printed on the substrate 147 using a flexography technique, i.e., a method in which a liquid alignment-layer-forming ink is applied on the resin plate 143 by the anilox roller, which contains a pattern of very fine dimples, or cells, corresponding to the pattern of the desired alignment layer to be formed, and then transferred from the resin plate onto the substrate 147. However, a drawback involved with using the foregoing flexography type of alignment layer printing apparatus is that it wastes a relatively large amount of the alignment layer ink during the process, thereby increasing LCD production costs.
The prior art alignment layer printing apparatus of FIG. 1B includes an ink jet head 150 for ejecting an alignment layer ink directly onto an LCD substrate 160. In this embodiment, the inkjet head 150 ejects the alignment layer ink directly onto the LCD substrate 160 through nozzles having orifices that eject the alignment layer ink onto the substrate in a pattern corresponding to the desired alignment layer 170.
FIG. 2 is an enlarged schematic perspective view of the prior art ink jet head 150 used in the prior art alignment layer printing apparatus of FIG. 1B, and FIGS. 3A and 3B are elevation views schematically illustrating a method for printing an alignment layer on an LCD substrate using the prior art ink jet head apparatus of FIG. 1B. Referring to FIG. 2, the ink jet head 150 includes a plurality of nozzles N1, N2, N3, N4, . . . , N2n−1 and N2n that are arranged in two parallel rows. The nozzles in the two rows are arranged such that the nozzles are spaced apart from one another by a selected interval, e.g., 750 microns (μm).
Referring to FIGS. 3A and 3B, in a first pass, or scanning session with the prior art apparatus, an alignment layer ink is ejected through the respective, spaced-apart nozzles of the ink jet head onto the substrate in the form of droplets, as illustrated in FIG. 3A, left, and the ink droplets are subsequently spread out and leveled with a wiping blade to form respective first alignment layer patterns, as illustrated in FIG. 3A, right. At this stage, since the spacing between the nozzles is relatively large, the alignment layer exhibits an undesirable variation in thickness, as illustrated in FIG. 3A, right. Then, in a second scanning session with the apparatus, the ink jet head is offset, or moved, relative to the substrate, such that the nozzles eject droplets of the alignment layer ink onto the substrate at positions intermediate of the positions at which the ink droplets were initially ejected during the first scanning session, as illustrated in FIG. 3B, left. These alignment layer droplets are also then spread out and leveled, as illustrated in FIG. 3B, right. However, as shown in FIG. 3B, right, even with this “two-session” ejection-and-leveling procedure, the alignment layer can still exhibit a variation in its thickness that results in the formation of undesirable longitudinal “stripes” in the alignment layer that can adversely affect the alignment of the liquid crystal molecules of the display, and hence, the quality of the images that it forms.